The Allen’s test is a quick, non-invasive medical procedure used to evaluate the blood circulation in the hand, acting as a simple screening tool. Healthcare professionals perform this assessment before procedures that might temporarily or permanently compromise the blood flow through one of the main arteries in the wrist. By checking the hand’s vascular supply, the test helps ensure the hand will continue to receive adequate oxygen and nutrients even if one vessel is affected.
The Purpose of Allen’s Test
The primary purpose of the Allen’s test is to assess the patency of the collateral circulation in the hand. This collateral circulation is a backup network of vessels, specifically the palmar arch, which connects the two main arteries supplying the hand: the radial artery and the ulnar artery.
The test is most commonly performed when a medical procedure requires accessing the radial artery, such as drawing blood for an arterial blood gas analysis or inserting an arterial line for continuous blood pressure monitoring. If the radial artery is compromised, a robust collateral circulation is needed to prevent tissue damage. The test confirms that the ulnar artery can independently provide sufficient blood flow to the entire hand, mitigating the risk of hand ischemia.
Performing the Assessment
The most common method used in clinical practice is the Modified Allen’s Test, which focuses on one hand at a time for greater efficiency. The patient is first asked to clench their fist tightly for about 30 seconds. This action forces the blood out of the hand, causing the palm to become pale or blanched.
While the patient maintains a tight fist, the examiner uses their fingers to apply firm pressure to both the radial and ulnar arteries at the wrist. This simultaneous compression temporarily blocks blood flow from both vessels. The patient is then instructed to open their hand, which will appear noticeably pale due to the lack of blood supply.
The examiner then releases the pressure on the ulnar artery only, while maintaining the compression on the radial artery. The assessment involves observing the palm’s color change and timing how quickly its normal pink hue returns. This timed observation assesses the capability of the ulnar artery to single-handedly supply the entire hand via the connecting palmar arch.
Interpreting the Results
The interpretation of the Allen’s test centers on the speed of color return to the palm. A rapid return of the pink color, generally within 5 to 15 seconds, is considered an adequate result, sometimes referred to as a “normal” or “positive” test. This speedy reperfusion indicates that the ulnar artery and the collateral circulation are fully open and capable of supplying the hand even if the radial artery is compromised.
If the hand remains pale for longer than 15 seconds, the result is considered abnormal, suggesting inadequate collateral blood flow. This outcome, often termed a “negative” test, means the ulnar artery cannot sufficiently take over the blood supply to the hand. In such cases, the radial artery should not be used for the planned procedure, as doing so carries a significant risk of causing ischemia or tissue death in the hand.